KVM: arm64: Allow no running vcpu on saving vgic3 pending table

We don't have a running VCPU context to save vgic3 pending table due
to KVM_DEV_ARM_VGIC_{GRP_CTRL, SAVE_PENDING_TABLES} command on KVM
device "kvm-arm-vgic-v3". The unknown case is caught by kvm-unit-tests.

   # ./kvm-unit-tests/tests/its-pending-migration
   WARNING: CPU: 120 PID: 7973 at arch/arm64/kvm/../../../virt/kvm/kvm_main.c:3325 \
   mark_page_dirty_in_slot+0x60/0xe0
    :
   mark_page_dirty_in_slot+0x60/0xe0
   __kvm_write_guest_page+0xcc/0x100
   kvm_write_guest+0x7c/0xb0
   vgic_v3_save_pending_tables+0x148/0x2a0
   vgic_set_common_attr+0x158/0x240
   vgic_v3_set_attr+0x4c/0x5c
   kvm_device_ioctl+0x100/0x160
   __arm64_sys_ioctl+0xa8/0xf0
   invoke_syscall.constprop.0+0x7c/0xd0
   el0_svc_common.constprop.0+0x144/0x160
   do_el0_svc+0x34/0x60
   el0_svc+0x3c/0x1a0
   el0t_64_sync_handler+0xb4/0x130
   el0t_64_sync+0x178/0x17c

Use vgic_write_guest_lock() to save vgic3 pending table.
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230126235451.469087-5-gshan@redhat.com

Documentation/virt/kvm/api.rst

@@ -8074,7 +8074,9 @@ NOTE: Multiple examples of using the backup bitmap: (1) save vgic/its
 tables through command KVM_DEV_ARM_{VGIC_GRP_CTRL, ITS_SAVE_TABLES} on
 KVM device "kvm-arm-vgic-its". (2) restore vgic/its tables through
 command KVM_DEV_ARM_{VGIC_GRP_CTRL, ITS_RESTORE_TABLES} on KVM device
-"kvm-arm-vgic-its". VGICv3 LPI pending status is restored.
+"kvm-arm-vgic-its". VGICv3 LPI pending status is restored. (3) save
+vgic3 pending table through KVM_DEV_ARM_VGIC_{GRP_CTRL, SAVE_PENDING_TABLES}
+command on KVM device "kvm-arm-vgic-v3".
 
 8.30 KVM_CAP_XEN_HVM
 --------------------

arch/arm64/kvm/vgic/vgic-v3.c

@@ -434,7 +434,7 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
 		else
 			val &= ~(1 << bit_nr);
 
-		ret = kvm_write_guest_lock(kvm, ptr, &val, 1);
+		ret = vgic_write_guest_lock(kvm, ptr, &val, 1);
 		if (ret)
 			goto out;
 	}